All subjects must meet the established diagnostic criteria for idiopathic hypereosinophilic syndrome: eosinophilia greater than 1,500/mm3 on two occasions at least 6 months apart, no known etiology for the eosinophilia despite careful clinical evaluation, evidence of end organ damage (histologic evidence of tissue infiltration by eosinophils and/or objective evidence of clinical pathology in any organ system that is temporarily associated with eosinophilia and not clearly attributable to another cause).